Liam Flanagan-Brown, 9 of Meriden, uses an interactive screen showing the rise in coastal flooding in the new global warming exhibit at the Peabody Museum in New Haven on Wednesday, January 30, 2013.

Photo: Brian A. Pounds

Liam Flanagan-Brown, 9 of Meriden, uses an interactive screen...

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Education coordinator Jim Sirch uses an interactive map with students from New Haven's Ross Woodward Classical Studies magnet school to teach about carbon dioxide emissions in the new global warming exhibit at the Peabody Museum in New Haven on Wednesday, January 30, 2013.

Photo: Brian A. Pounds

Education coordinator Jim Sirch uses an interactive map with...

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Ross Woodward Classical Studies eighth grader Davonne Rountree, of New Haven, works on a class assignment in the new global warming exhibit at the Peabody Museum in New Haven on Wednesday, January 30, 2013.

Photo: Brian A. Pounds

Ross Woodward Classical Studies eighth grader Davonne Rountree, of...

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Seasons of Change: Global Warming in Your Backyard, discusses the local effects of climate change at the Peabody Museum in New Haven.

Photo: Brian A. Pounds

Seasons of Change: Global Warming in Your Backyard, discusses the...

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An interactive exhibit illustrates how storm flooding will effect the city of Boston with rising sea levels due to climate change at the Peabody Museum in New Haven.

In his inaugural address, President Barack Obama made climate change a priority of his second term. It may be too late.

Within the lifetimes of today's children, scientists say, the climate could reach a state unknown in civilization.

In that time, global carbon dioxide emissions from burning fossil fuels are on track to exceed the limits that scientists believe could prevent catastrophic warming. CO2 levels are higher than they have been in 15 million years.

The Arctic, melting rapidly and probably irreversibly, has reached a state that the Vikings would not recognize.

"We are poised right at the edge of some very major changes on Earth," said Anthony Barnosky, a UC Berkeley professor of biology who studies the interaction of climate change with population growth and land use. "We really are a geological force that's changing the planet."

Maple syrup production, for example, is moving further and further north. By the end of this century, it will come almost entirely from Canada, experts say. And that goes for a host of other quintessentially New England products, from lobsters to cranberries to blueberries.

"We don't think about how the changing climate will change the way we live," said Yale professor David Skelly, who had a role in the exhibit. "Rather than arguing about whether this storm or that storm has been caused by climate change, this is about what we might expect in the future."

He said that a past professor at Yale, the late George Evelyn Hutchinson, was warning as early as 1949 that increasing levels of carbon dioxide in the planet's atmosphere would inexorably lead to a warmer world. Hutchinson taught at Yale from 1928 to 1971.

"He was one of the first people to put the pieces together," Skelly said.

The potential changes ahead are dramatically described at the Peabody exhibit, Seasons of Change: Global Warming in Your Backyard," which runs through Feb. 24.

New south

The Arctic melt is occurring as the planet is just 0.8 degree Celsius (1.4 degrees Fahrenheit) warmer than it was in pre-industrial times.

At current trends, the Earth could warm by four degrees Celsius in 50 years, according to a November World Bank report.

The coolest winter months would be much warmer than today's hottest summer months, the report said. "The last time Earth was four degrees warmer than it is now was about 14 million years ago," Barnosky said.

Experts said it is technically feasible to halt such changes by nearly ending the use of fossil fuels. It would require a wholesale shift to renewable fuels that the United States, let alone China and other developing countries, appears unlikely to make.

Indeed, many Americans do not believe humans are changing the climate.

"You can't make a thermometer tell you it's hotter than it is," said Hayhoe, who with her husband, a linguist and West Texas pastor, has written a book on climate change addressed to evangelicals.

"And it's not just about thermometers or satellite instruments," she said. "It's about looking in our own backyards, when the trees are flowering now compared to 30 years ago, what types of birds and butterflies and bugs we see that ... used to be further south."

Robins are arriving two weeks early in Colorado. Frogs are calling sooner in Ithaca, N.Y. The Sierra Nevada snowpack is melting earlier. Cold snaps like the one gripping the East still happen, but less often.

The frost-free season has lengthened 10 days in Connecticut, to 21 days in California, according to the draft climate assessment.

Scientists are loath to pin a specific event such as Sandy or floods in England to global warming.

But "the risk of certain extreme events, such as the 2003 European heat wave, the 2010 Russian heat wave and fires, and the 2011 Texas heat wave and drought has ... doubled or more," said Michael Wehner, a staff scientist at Lawrence Berkeley National Laboratory and co-author of the climate assessment report. "Some of the changes that have occurred are permanent on human time scales."

The continental United States last year was the hottest in 188 years that records have been kept. Globally, the first 12 years of the 21st century were among the 14 warmest ever.

The pine bark beetle, long held in check by winter freezes, is epidemic over millions of acres of forests from California to South Dakota.

Darker oceans

Oceans, which absorb CO2, have increased in acidity, damaging coral reefs, shellfish and organisms at the bottom of the food chain. Washington state shellfish growers have seen major failures in oyster hatcheries because the larvae don't form shells.

A National Research Council report this month said such changes in ocean chemistry in the geologic past were accompanied by "mass extinctions of ocean or terrestrial life or both."

A key question is when greenhouse gas emissions might reach a tipping point, where changes become self-reinforcing and out of human control.

Arctic sea ice reflects the sun. As it melts, the dark ocean absorbs more solar heat, raising temperatures. Similarly, the Greenland ice sheet is melting rapidly, reducing reflectivity, and possibly speeding up the melting of the West Antarctic ice sheet. The northern permafrost is thawing, with the potential to release methane, a potent greenhouse gas, and CO2 stored in soils. These can produce sudden, so-called non-linear changes that are hard to predict.

"We could be at a tipping point where the climate just abruptly warms," said Mark Z. Jacobsen, director of Stanford University's atmosphere/energy program. An Arctic melting "would make it more difficult for the Northern hemisphere to cool down, so Greenland would be next. Greenland stores about five to seven meters of sea level. The Antarctic in the Southern Hemisphere stores another 65 meters of sea level. That would take longer, but even that is starting to crack."

UC Berkeley's Barnosky said tipping points could come earlier than anticipated when factoring in population growth and land use.

More than 40 percent of the earth's land surface has been covered by farms and cities. Much of the rest is cut by roads. By 2025, the percentage could reach half, a level that on smaller scales has led to ecological crashes.

"It's just sort of simple math: the more people, the more footprint," Barnosky said. "If we're still on a fossil fuel economy in 50 years, there is no hope for doing anything about climate change. It will be here in such a dramatic way that we won't recognize the planet we're on."

Not all climate scientists are so gloomy. Ashley Ballantyne, a bioclimatologist at the University of Montana who studies paleoclimate records, said the climate has always changed, with ice ages, warmings and mass extinctions. He said at current CO2 concentrations, the Arctic and Greenland are likely to become ice free, as they were four million years ago.

Polar bears are poorly adapted to such conditions, he said, "but it wasn't bad for boreal trees. They were quite happy."

The oil hole

An international political consensus set as a danger zone two degrees of warming, expected in 25 years on current trends when atmospheric concentration of CO2 reaches 450 parts per million. It is now 400 parts per million.

Two degrees is "an arbitrary number," said Alan Robock, director of the Center for Environmental Prediction at Rutgers University. "On our current path, we will go zooming way past that."

Climatologist James Hansen, head of NASA's Goddard Institute for Space Studies and activist Bill McKibbon, founder of 350.org, believe the only way to preserve the climate humans are used to is to cut CO2 concentrations to 350 parts per million, last seen around 1988.

McKibbon and Hanse propose a tax on fossil fuels at their source, to be reimbursed to all U.S. residents. U.S. Sen. Bernie Sanders, I-Vt., plans to propose that in a "fee and dividend" scheme modeled on Alaska's oil royalty rebates to state residents.

It would have to be a big tax, McKibbon said, "that drives up the price quickly. Maybe you go to the pump someday and you're paying what people in Europe pay for gasoline, which is good, because then it reminds you every time you go to the pump that you don't really need a semi-military vehicle to go to the grocery store."

McKibben's 1989 book, "The End of Nature," did much to alert the world on the perils of climate change. Many of its predictions are already becoming reality.

Jacobson maintains that wind and solar could power the world many times over. He calculated that the world would need to install 1.7 billion solar rooftops and four million wind turbines.

Jane Long, chair of the California Council on Science and Technology, said any such conversion would be costly and difficult at best. Still, she said, "one way to get out of the hole is to stop digging."

Carolyn Lochhead is the San Francisco Chronicle's Washington correspondent. clochhead@sfchronicle.com